This invention generally relates to a method and apparatus for providing an improved DC power plant system capable of delivering emergency DC power to telephone equipment or other loads. More specifically this invention relates to a DC power back-up system using AC driven rectifiers connected to charge back-up batteries and power DC telecom equipment.
In telephone switching equipment, communication and computer equipment, and in many other applications, the need for an uninterrupted source of DC power is critical. Rectified commercial AC power is typically used as the primary source of DC power for such equipment. The DC power from the rectifiers is delivered to the back-up batteries and the load through a load bus and returned to the rectifiers along a ground bus.
To avoid any interruption or outage in power service, it is common practice to employ a battery back-up system for the primary DC source. Back-up battery systems typically include strings of batteries or cells connected in parallel with the primary DC source and the load. Some systems are also equipped with AC generators to further back up the AC line power. In the event of a drop in the DC load bus voltage below a predetermined threshold, the back-up battery supplants or supplements the primary source of DC power. The battery back-up continues until either AC line power is resumed or the AC emergency generator is activated to supply AC power to the rectifiers. Back-up battery systems are designed to replace the primary DC power source for a predetermined period of time. Within that period of time, the resumption of primary AC line power is expected to occur.
In conventional back-up battery systems, the nominal system load bus voltage is typically dictated by battery characteristics. For example, in a telephone switching plant, back-up batteries are commonly employed which each have a design float charge cell voltage of 2.17 volts, for optimum health of the battery cell. Twenty-four cells are typically connected in series to form a string resulting in a nominal load bus voltage of approximately xe2x88x9252.10 volts. A bank of strings supplies the necessary back-up DC power.
When AC line power is initially turned on or AC emergency power is activated in place of the AC line power, the back-up batteries tend to draw an excessive amount of current since these are placed in parallel with the DC load. This design architecture of a typical back-up battery system presents a number of disadvantages. Partially or fully discharged batteries, due to their electrochemical constitution, will draw an excessive amount of current in order to recharge themselves as quickly as possible. All batteries, no matter the time spent on discharge, will initially demand a high amount of recharge current from the rectifiers or the primary DC power source. Consequently, upon the return of AC power to the rectifiers the initial current to recharge the batteries is must be counted as connected load. Emergency AC generators for use with central office DC power plants are thus routinely sized to provide for this one-time power drain, which occurs at most but occasionally.
In addition, the paralleling of all available rectifiers or rectified DC power sources forces them all to share and satisfy the initially high connected load leading to inefficient operation of the AC to DC conversion by the rectifiers. Therefore, the two main disadvantages of the current systems are:
1. Over subscribed kilowatt sizing for installed AC emergency generator power.
2. Less efficient operation of all rectified DC power sources.
This invention provides a system to improve the efficiency of the utilized DC rectifiers while still providing standby spare rectifiers that automatically come online in the event a load rectifier fails. In addition, because of the specific rectifier arrangement of the invention, the standby emergency power source can be reduced in size to satisfy the initial connected current flow from only the rectifiers connected to the load and back-up batteries rather than all the available rectifiers.
The invention achieves the improved system by segregating the available rectifiers into two groups. A first group of load rectifiers for delivering and satisfying normal charging operation of the back-up batteries and the load and a second group of spare rectifiers that serves as spares for selective connection to the system when one of the load rectifiers fails.
Both disadvantages are addressed by the invention in that the segregation of the load rectifiers from the spare rectifiers provides alternative options that allow the DC power plant to operate more efficiently by intentionally failing to provide sufficient DC power to satisfy the start-up current demands imposed by the parallel connected back-up batteries and the load and allowing an initial small amount of DC voltage drop to occur on the load bus. This condition lasts only briefly, but permits one to reduce the startup current to thus reduce the power capacity or sizing of the emergency generator, while enabling a spare rectifier to be activated to supply DC power when a load rectifier fails.
Segregating the load and spare DC power sources of the DC power plant thus makes more efficient use of the load rectifiers, and generates capital cost savings. The cost savings are realized by decreasing the size of the emergency power generator so as to cover only the current demanded by the load rectifiers, and not the spare rectifiers as are common in conventional systems.
Accordingly, it is the object of the invention to provide a system and method, which improves the operating efficiency of standby emergency DC power plant systems.
Another object of the invention is to provide a system and method, which lowers the costs associated with standby emergency AC generators used to provide power to DC power plant systems.
Another aspect of the invention comprises a method for more efficiently operating a DC power plant system utilizing battery back up and a standby emergency power source for use with telephone switching equipment or other loads.
The invention and its particular features and advantages will become more apparent from the following detailed description considered with reference to the accompanying drawings.